1. Field of the Invention
The present invention relates in general to a method of analyzing an electroretinogram, and more particularly to a method of extracting or separating an oscillatory potential component from the electroretinogram and measuring various parameters of the separated OP component, by processing the detected electroretinogram, based on electrophysiological knowledge and findings, without conventionally available electrical, mechanical or manual processing techniques.
2. Discussion of the Prior Art
An electroretinogram (hereinafter abbreviated as "ERG" when appropriate) is a graphic record of the manner in which a potential developed by retina cells of the eye in response to a flash light stimulus varies with time. The potential is developed between the cornea and the forehead (or chin or cheek). Basically, the ERG consists of three major components, i.e., an "a" wave, a "b" wave and an oscillatory potential wave (hereinafter abbreviated as "OP wave", or referred to as "OP component" or "OP component wave" as appropriate), as shown in FIG. 1. The ERG response to a light stimulus is considered a sum of potentials which are induced by various cells of the retina of the eye. Described more specifically, the "a" wave is a result of a response of the visual cells and the "b" wave is a result of a response of the Muller cells, while the OP wave is a result of a response of the Amacrine cells.
Since it is impossible to directly observe or record responses of the various retina cells of the human eyes, an ERG or electroretinography is a very effective way of obtaining data representative of the functioning conditions of the retina cells. In recent years, therefore, the ERG is widely utilized for many varied clinical purposes, for example, for diagnosis or determination of ocular pathology such as opaque intermediate media or vitreous, and retinopathy, and for inspection of the visual function of infant. In particular, it is known that the OP component of an ERG detected on a subject suffering from diabetes, Behcet syndrome or other diseases has a tendency of declining or disappearing even in a relatively early stage of development of such diseases. Accordingly, the OP component of the ERG is useful for finding such diseases at a relatively initial period of development thereof.
There are known some methods of separating and analyzing the oscillatory potential or OP component from a detected ERG. For instance, a filter is used to process a detected ERG response, and different time constants of the filter are used to detect the "a" and "b" waves and to detect the OP component, so that the OP component as distinguished from the "a" and "b" waves is amplified, as indicated in FIGS. 2(a) and 2(b). The obtained OP component provides an aid for the empiric determination of diseases, based on a relation between the waveform of the OP component and the diseases. The OP component of the detected ERG is manually processed to measure various characteristic parameters of the OP component wave, as indicated in FIG. 3, such as: amplitudes O1, O2, etc. which are distances between straight lines connecting adjacent negative peaks of the OP wave, and positive peaks of the same; latency times Dp, Db between a moment of light stimulation to the retina and the first positive and negative peaks of the OP wave, respectively; and time durations T1, T2, T3, t1, t2 between the adjacent peaks. An alternative method to measure the OP component is accomplished by linearly interpolating midpoints of the amplitude of the OP wave and thereby separating the "b" wave, and subtracting the "b" wave from the detected or measured ERG.
However, the known electric, mechanical or manual methods of extracting and analyzing the OP component or wave of an ERG are not accurate enough for objective determination or analysis in clinical diagnosis, and suffers from several problems in their practice. Stated in greater detail, the "a" wave, "b" wave and OP component wave of an ERG which have different latency times (times delays) after the moment of light stimulation overlap each other in a complicated fashion in the axis of time. Further, in the power spectrum of a typical ERG, the OP wave and "a" and "b" waves which are signals having peaks in a relatively narrow frequency band in the neighborhood of one hundred and several tens of Hz, overlap each other, in the axis of frequency, as indicated in FIG. 4. Hence, the relatively simple conventional methods are inherently incapable of accurately extracting and analyzing the OP component. While there have been attempts in the field of engineering to analyze the ERG in terms of the frequencies of the components, such attempts are not practically accurate and reliable from the clinical or physiological standpoint, for objective analysis of the ERG and determination of the parameters of its OP component which represent ocular pathology or retinopathy and related diseases.